What to play for rhythm behind diminished chords?

Our acoustic player/singer wrote a jazzy sounding song recently that switches keys 3 times in the middle of the song and has several diminished chords. I'm not really sure what to play behind the diminished chords and still sound right, but minor and major chord substitutions don't work.

Do I have to play the actual chord or are there some substitutions that will work for diminished chords? Thanks

Our singer plays the acoustic and I play rhythm only. When he plays a specific chord I try to play something different to harmonize, and try not to play in the same area of the neck or sometimes not even the exact same chord as him. An example would be playing a D up the neck behind him strumming a Bm7th, stuff like that. The Bm7th contains all the notes of the D so it harmonizes and sounds cool.

The other night when he broke out his new song I wasn't sure if there were any options for playing rhythm behind the diminished chords, other than playing the actual diminished chords themselves.

Our singer plays the acoustic and I play rhythm only. When he plays a specific chord I try to play something different to harmonize, and try not to play in the same area of the neck or sometimes not even the exact same chord as him. An example would be playing a D up the neck behind him strumming a Bm7th, stuff like that. The Bm7th contains all the notes of the D so it harmonizes and sounds cool.

The other night when he broke out his new song I wasn't sure if there were any options for playing rhythm behind the diminished chords, other than playing the actual diminished chords themselves.

Maybe arpeggiate something? Keep in mind that all diminished chords connect as well. Move a diminished chord shape up three frets to find another diminished chord with the same notes. If you have something like this E diminished seventh chord:

e-3-B-2-G-3-D-2-

You can play any of these, they have the same notes:e-6-B-5-G-6-D-5-

e-9-B-8-G-9-D-8-

e-12-B-11-G-12-D-11-

But those likely wouldn't sound great while he's playing a different voicing, so I'd go the route of arpeggiating or some kind of little lead lick.

if the chord substitutions and inversions dont quite achieve the sound you are looking for, try play a dominant chord down a half step from the diminished chord's root, that will create an altered dominant sound which are used often during modulation (almost religiously in jazz)